In construction vehicles, such as track type tractors or excavators, it is common to employ separate hydrostatic transmissions for driving individual ground engaging devices such as tracks or wheels. Each hydrostatic transmission typically includes a fluid pump adapted to supply pressurized fluid to a respective drive motor for controlling the speed and direction thereof. Each hydraulic motor is in turn mechanically connected to a respective ground engaging device for propelling the vehicle.
The speed and direction of the vehicle is controlled in response to the flow rate and direction of pressurized fluid applied to the drive motors. Rectilinear vehicle travel is achieved by operating the drive motors at the same speed and in the same direction. Conversely, cornering operations can be effected by varying the speeds of the individual drive motors. Typically, separate control levers or pedals are provided for controlling the speeds of the individual drive motors. For example, in an excavator, left and right control levers are provided for controlling the speed and direction of left and right drive motors. The control levers are manually movable between a plurality of forward and reverse positions and are normally biased to a neutral position. In most applications, the control levers are connected to either electro-hydraulic or hydro-mechanical control systems which control the rate and direction of fluid flow to the motors in response to the position of the control levers.
In many instances, it is desirable to maintain the motors at a constant speed for an extended duration. In existing systems, such an operation requires the operator to maintain the control levers at fixed position for an extended period of time. As should be apparent, this can become tiresome and fatiguing for the vehicle operator.
The present invention is directed to overcoming the problem as set forth above.